This invention relates to a process for the manufacture of resol type resin-coated refractory granules, especially sand, especially adapted for use casting core molds, said coating resin being characterized by a minimum nitrogen content.
It has been commonly known to coat granular refractory material, especially sand, with novolac type phenolic resin added with 10 - 15 wt. % of hexamine as hardening agent, in order to prepare casting molds and cores for use in the shell molding technique as a most broadly prevailing means.
While the use of hexamine as hardening agent for novolac type phenolic resin accelerates substantially the hardening velocity thereof, thus representing a superior and advantageous feature as the hardener, it generates a substantial amount of gaseous nitrogen through thermal decomposition of the hexamine by contact with higher temperature molten metal in the course of moulding of cast iron or cast steel products, thereby inviting numerous gas defects, such as pin holes and blow holes. The gaseous nitrogen frequently includes vaporized amine which deteriorates in the ambient atmosphere and gives out a noxious smell during the preparation stage of the molds and cores as well as the pouring stage. Improvements in this respect are strongly desired for the prevention of industrial pollution.
Various proposals have therefore been made for the production of sand-coating resin including no, or only such a limited amount of nitrogen as to provide substantially no harmful effect. As an example, paraformaldehyde has been used as a hardening agent for novolac type phenolic resin, since this substance can act as a supply source of formaldehyde and harden the phenolic resin upon heating.
However, when refractory sand coated with paraformaldehyde containing is used for the preparation of shell molds and cores, the molds and cores frequently are distorted or even damaged due to defective thermal hardening of the coated resinous layer, when the shells are taken out of their respective master molds. In addition, a smoke of unhealthy poisonous gases is released during the hardening operation.
Further, it is also known to use resol type phenolic resin as the hardener for novolac type phenolic resin and conditioned with a catalyst such as an alkali or alkali earth metal in the form of oxide or hydroxide thereof. With the use of resol type phenolic resin conditioned with catalyst, preferably alkali metal oxide or the like, a grave drawback may be encountered in that the resin has a strong tendency to absorb moisture due to an appreciable residual quantity of the catalyst in the resin substrate. This tendency leads to weakening of the strength of the molds prepared therefrom and becomes rather appreciable in a high moisture environment as met in Japan. By absorbing an appreciable amount of moisture, the coated sand, originally having an easily flowing state, conglomerates which means a substantial drawback in the art. Due to the high viscosity of this kind of resin, highly fluidous resin coated sand can only be prepared through a long kneading step, resulting in a high difficulty of practical utilization thereof.
In consideration of the above facts including the adverse effect of the residual alkali metal and with the intentional utilization of self-hardenability of resol type phenolic resin, intensified attention has been directed by those skilled in the art towards the use of resol type phenolic resin conditioned with an ammonia catalyst.
The term "resol type phenolic resin" as used throughout the present specification and in the appended claims means such a resin which can be prepared from 1 mole of phenol and at least 1 mole of formaldehyde which are reacted with each other in the presence of an alkaline catalyst. This resin can be classified into two general classes. The first one is such a resin normally called "resol" which can be prepared in the presence of said alkali metal hydroxide or the like catalyst. The second one is such a resin normally called "ammonia-catalyzed resol" which can be prepared in the presence of such a catalyst as ammonia, primary amine, such as, preferably, monoethyl amine, monomethyl amine, secondary amine, such as, preferably, diethyl amine, dimethyl amine, or the like. The said resol is obtainable in the form of a viscous liquid and has water solubility and hydrophilic properties. It is soluble in organic solvents such as alcohol, acetone, and the like, and utilized frequently and broadly as a varnish.
On the other hand, ammonia-catalyzed resol can be obtained not only in the form of a viscous liquid, but also a solid, depending upon the reacting conditions; and, as a specific feature, it can be hydrophobic and soluble in organic solvents such as alcohol, acetone and the like.
The solid ammonia-catalyzed resol has the following several predominant utilities over the liquid state resin.
1. It is easily treated in various processing stages due to its solid state.
2. Resol resin has self-condensability, thus being highly limited in its storage term and conditions. Liquid state resol resin can generally be stored only for approximately three months, while solid state resol can be stored as long as 6 months in its stabilized condition.
However, the following substantial difficulties have been met in the preparation of solid state resol.
In order to guarantee the stabilized storability even in a high temperature environment as above specified, the resin must have its softening point ranging between 80.degree. and 85.degree.C and, for assuring such softening point, the final heating temperature of the resin should be higher by about 20.degree.C than the above specified softening point. Due to self-condensability of the resol resin, when heated to approximately 100.degree.C, the condensation of the resin progresses in the mode of a chain reaction, and the reaction velocity rises to an extremely accelerated rate in such manner that with a 1.degree.C temperature rise, it increases about ten times; thus a desired velocity control is practically impossible. Reports can be found from the literature of the known resin molding or laminating process such that the final processing temperature of resol resin is limited to approximately from 70.degree.C to 75.degree.C, for easy manufacture of the final products.
In order to manufacture resol resin on a large scale, the reaction vessel generally has a volumetric capacity of 3,000 - 10,000 liters. In this case, it is inhibitingly difficult to quench such a large amount of the resin, once heated up to approximately 100.degree.C, for approximately 30 minutes on an industrial scale in order to prevent the self-condensation. Or more specifically, resins are generally heat-insulating and thus it is highly difficult to effectively cool resinous products from outside until the core portions thereof have been cooled down. In fact, the self-condensing reaction progress rapidly within the interior parts of the products under cooling, inviting an exothermic reaction in an abrupt and explosive manner and in the solidifying direction.
In addition to various and profound difficulties as met in the manufacture of solid state resol, it should be further noted that difficulties are encountered in the manufacture of solid resol resin devoid of aqueous moisture content, for assuring a favorable and efficient coating ability thereof, as well as an improved storage stability.
As an example, if removal of the aqueous content from the reaction product simply by exposing it under normal pressure to a high temperature atmosphere is tried, the thus caused temperature rise of the product will further accelerate the reaction, thereby abruptly reducing the number of methylol radicals contained therein and subjecting the resin-coated granules to a retarded resin-hardening velocity. This will lead naturally to a corresponding reduction in the binding strength of the molds or the like final products prepared therefrom.